ive just managed to get this working by half following what you did and adding in some bits.
ok
1)install drivers in the same way you did, but after configure,itll do it for 2 snes pads just accept that for now,
2 ) add your db9_gpio_rpi to modules as you did ( -the map)
3) added a db9.conf (db9_gpio_rpi map=4,0)file to modprobe.d/ folder
that was it
im pretty new to this so hop that does it for you as well ?